1. Field of the Invention
The present invention relates to an impeller used for a radial turbine such as a micro gas turbine, an expander turbine or a supercharger.
2. Description of the Related Art
An impeller used for a radial turbine, such as a micro gas turbine, an expander turbine or a supercharger is generally constituted by a plurality of blades; i.e., rotor blades; and a main disk provided with these rotor blades.
FIG. 5 is a front view of part of a prior art radial turbine impeller. As shown in FIG. 5, the impeller 110 is generally circular, and a plurality of rotor blades 400 are arranged on a rotary axis 120 of the impeller 110 generally at equal intervals in the circumferential direction. Paddle-like scallops 300 are formed between every two adjacent rotor blades 400 in the vicinity of the outer circumference of a main disk 200. As is apparent from FIG. 5, the scallop 300 is formed between a negative pressure surface 410 of the rotor blade 400 and a positive pressure surface 420′ of the rotor blade 400′ adjacent to the former. The scallops 300 are formed by cutting off the main disk 200 along the rotor blade from the outer circumference of the main disk 200 to a predetermined distance. In the main disk 200 in which the scallops 300 are formed, a minimum radius portion from the rotary axis 120 of the impeller 110 to an outer edge of the scallop 300 is located generally at a center between the two rotor blades 400 and 400′. Accordingly, the scallops 300 are symmetric in the left/right direction relative to the minimum radius portion. The scallops 300 serve to reduce a centrifugal force and a moment of inertia in the impeller 110.
FIG. 6a is a perspective view of the prior art radial turbine impeller. As shown by arrows F1 and F2, a fluid enters the impeller 110 in the vertical direction relative to the rotary axis 120 of the impeller 110 and then flows out from a turbine exit 160 in the parallel direction relative to the rotary axis 120. However, as a gap is formed between a casing (not shown) and a back surface of the impeller 110 when the scallop 300 is formed, a leakage FR, flowing from a positive pressure surface 420 to the negative pressure surface 410 is formed. To reduce the leakage, for example, in Japanese Unexamined Patent Publication (Kokai) No. 10-131704, a radial turbine impeller is disclosed, having scallops, each being asymmetric in the left/right direction so that the minimum radius portion of the scallops 300 are deviated, from a center of an area between the adjacent two blades, to be closer to the negative pressure surface of the blade.
However, in the prior art radial turbine impeller and the radial turbine impeller disclosed in Japanese Unexamined Patent Publication (Kokai) No. 10-131704, another problem occurs due to the scallop 300 formed by cutting off the main disk 200. This problem will be explained with reference to FIGS. 7a, 7b, 7c and 6b. In this regard, FIGS. 7a, 7b and 7c are an illustration of part of the prior art radial turbine impeller (a meridian plane), a sectional view taken along a line A—A in FIG. 7a as seen from upstream in the flowing direction, and a sectional view taken along a line B—B in FIG. 7a as seen from upstream in the flowing direction, respectively; and FIG. 6b is a side sectional view of the prior art radial turbine impeller. As shown in FIG. 6b, a flow F1 of the fluid flowing into the impeller 110 impinges on the edge of the scallop 300, causing a secondary flow FA (FIG. 7a) on the negative pressure surface 410 rising toward a rotor blade exit shroud 450, and a secondary flow on a surface of a hub 150 directing to the negative pressure surface 410. Thereby, as shown in FIG. 7b, corner vortices 500 generate in an area on the negative surface 410 of the rotor blade 400 closer to the hub 150. Such corner vortices 500 are low-energy fluids and gather together in an area closer to the shroud 450 of the negative pressure surface 410 in the vicinity of the exit of the rotor blade 400 (FIG. 7c). Thereby, the uniformity of the flow is disturbed to lower the effect of the turbine.
According to the radial turbine impeller disclosed in Japanese Unexamined Patent Publication No. 10-131704, it is possible to prevent the efficiency of the turbine from lowering due to the leakage occurring on the back surface of the impeller. However, as this impeller is not formed so that part of the scallop is adjacent to the negative pressure surface 410, it is impossible to prevent the efficiency of the turbine from lowering due to the generation of the corner vortices as in the prior art radial turbine impeller.
Accordingly, an object of the present invention is to provide a radial turbine impeller which prevents the efficiency of the turbine from lowering caused by the impingement of fluid onto the edge of the scallop.